1. Technical Field
The present invention relates to an electro-optical device including light emitting elements such as organic EL (electro luminescent) elements, and to an electronic apparatus including such an electro-optical device.
2. Related Art
An OLED (organic light emitting diode), that is, an organic EL element can be exemplified as a thin and lightweight light emitting source. The organic EL element has a structure in which at least one layer of an organic thin film including an organic material is interposed between a pixel electrode and an opposing electrode. The pixel electrode functions as a positive electrode, and the opposing electrode functions as a negative electrode, for example. When a current flows between both of them, electrons and holes are recombined in the organic thin film. As a result, the organic thin film or the organic EL element emits light.
As such an organic EL element and the image display device including the same, those disclosed in JP-A-2007-133283 are known.
The organic EL element as described above is driven by a drive circuit with an appropriate configuration. As an example of the drive circuit, there can be exemplified a drive circuit which supplies a current flowing between a drain and source of a drive transistor to the organic EL element in accordance with a gate potential of the drive transistor. In this case, it is possible to adjust the light-emission luminance of the organic EL element by adjusting the gate potential.
However, such a drive circuit has various problems to be solved. For example, one of the problems is a variation in characteristics such as mobility or a threshold voltage of the drive transistor. The image display device as described above is generally provided with multiple organic EL elements and the drive circuit including the drive transistors provided for each organic EL element. However, if each of the characteristics of the plurality of drive transistors varies due to variations in various parameters for the manufacturing process, a variation also occurs in the adjustment of light-emission luminance of each organic EL element. As a result, it becomes difficult to improve qualities of displayed images.
A technique relating to such a problem is disclosed in above-mentioned JP-A-2007-133283. That is, JP-A-2007-133283 discloses the technique to execute a mobility compensation operation for a drive circuit with a configuration in which a light emitting control transistor, a drive transistor, and an organic EL element are connected in series to each other and a data writing transistor is connected to a gate of the drive transistor. This mobility compensation operation according to JP-A-2007-133283 is performed between the following two operations: (1) an operation of writing a data potential to the gate of the light emitting control transistor (writing operation) while the light emitting control transistor is turned off and the data writing transistor is turned on; and (2) an operation of supplying a current of the drive transistor to the organic EL element (light emitting operation) while the light emitting control transistor is turned on and the data writing transistor is turned off. Specifically, the mobility compensation operation includes a step of performing (3) an operation of causing a current in accordance with the mobility to flow through the drive transistor while both the light emitting control transistor and the data writing transistor are turned on (refer to the time period between “T6” and “T7” in FIG. 4 or [0031] of JP-A-2007-133283).
The above-mentioned operation of (3) causes a source potential of the drive transistor to rise due to the current flowing therethrough, which results in a decrease in a gate-to-source voltage. However, since the damping of the current depends on the mobility of each drive transistor, performing the operation of (3) during the same time period for each drive transistor causes each gate-to-source voltage for respective drive transistors to have a value in accordance with the magnitude of the mobility of each (that is, the variation in the mobility characteristic of each drive transistor can be compensated).
However, the technique disclosed in JP-A-2007-133283 has the following problem. That is, it is extremely important to exactly manage the time period during which the operation of (3) is performed in order to preferably carry out the operation. In other words, the timing at which the light emitting control transistor is turned on, which distinguishes between the above-mentioned operations of (1) and (3), and the timing at which the data writing transistor is turned off, which distinguishes between the above-mentioned operations of (3) and (2), should be managed exactly for each drive circuit. If not, the damping of the current unfavorably varies for each drive transistor. That is, the current is excessively damped for some drive transistors and insufficiently damped for other drive transistors. As a result, a desired effect of mobility compensation cannot be secured.
However, it is difficult in general to exactly manage such timings. For example, when the drive circuit has a matrix-shaped arrangement, it is necessary to supply a control signal for turning off a data writing transistor arranged in a row to the data writing transistor. However, parasitic capacitance and parasitic resistances included in a supply line of the control signal may cause a distortion of the waveform of the signal as it goes through the supply line. As a result, there is a danger that a difference may occur between the opening and closing timings of the data writing transistors in the first and the last drive circuits in the row.